It may be helpful to notify users who may have forgotten to purge deleted mail (or who are new to Evolution and don't know that mail marked as trash has not been deleted or even moved yet) that they should do so (and/or offer to do the purging for them, and/or offer to do the purging automatically in future).

An edited except from the discussion on the mailing list, at http://mail.gnome.org/archives/evolution-list/2010-September/msg00116.html :
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> > IMHO, the ability to purge mailboxes should be more visible. Maybe
> > evolution should warn that a mailbox is full of 90% deleted messages
> > (which was my case).
> 
> Something perhaps along the lines of this pseudo-code:
> 
> if( num_trash_msgs >= 1000 && num_trash_msgs / total_msg_count >= 0.6 ) then
>     show_dialog( "Your mailbox contains a lot of trash messages that have not been deleted yet. Do you wish to purge trash messages now?", "Purge", "Cancel", purgeTrashCallback() )
> end if
> 
> > since a significant number of the queries to
> > this list involve the fact that people don't see deleted messages.
> 
> Yep, often this involves thinking that something is deleted when it's
> just marked as trash. The above _may_ go some way towards helping
> prevent that.

Not unreasonable. Someone should file an RFE suggesting it. In fact there are several possible conditions:

* High proportion of messages marked for deletion, for mailboxes with more than some threshold of total messages (i.e. your test).

* Mailbox is approaching a size limit. For local mboxes, 2.32 apparently will correct this by using 64-bit indexes. For remote mailboxes, IMAP servers have a soft/hard quota system but I'm not sure how Evo deals with it currently. No idea about Exchange, Groupwise etc.

* Running low on space on the filesystem where .evolution lives (for local mailboxes, whether mbox or maildir).

* Old unexpunged messages (currently you can configure automatic expunging for these, but some people might prefer to be warned).

* High absolute number of unexpunged messages, irrelevant of deleted:non-deleted ratios (e.g. maybe more than 2000 deleted messages?).
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When the check could potentially happen:
* On Send/receive.
* On application start.
* On application close (although personally this is my least favorite option - the last thing I want when trying to finish up at the end of a day is to be prompted for something, or to have to make a decision).
* On deleting a message.